Arch M. Reid

Syndepositional shallow-water precipitation of glauconitic minerals

Abstract Numerous studies have demonstrated that glauconitic minerals predominantly form in water depths of mid-shelf to upper slope in modern oceans. These areas tend to have slow sedimentation rates, another commonly cited requisite for glauconitic mineral precipitation. Cambro-Ordovician strata from the southwestern US are rich in glauconitic minerals. Stratigraphic, sedimentological, and petrographic constraints indicate that the glauconitic minerals are autochthonous. In marked contrast to the modern environments of deposition, these Cambro-Ordovician strata formed under very shallow-water to tidal-flat conditions. The trough cross-stratified deposits of the most glauconitic mineral-rich accumulations (glaucarenites) indicate a high energy environment and probably a normal to high rate of sedimentation. The presence of fibroradiated rims of glauconitic minerals on glauconitic mineral pellets, echinoderm fragments, and quartz grains demonstrates that the Cambro-Ordovician glauconitic minerals precipitated on or in close proximity to the sea floor and prior to calcite precipitation. Consequently, glauconitic minerals must have formed under markedly different conditions in the lower Paleozoic than they do today. Thus, the occurrence of glauconitic minerals in the rock record cannot be used a priori as an environmental indicator of either mid-shelf and deeper water and/or a slow rate of sedimentation.

Apollo 15 green glasses.

Abstract Apollo 15 breccia 15427 and soils 15101, 15261 and 15301 contain abundant spheres and fragments of a green glass that is remarkably constant in composition. The glass is rich in Fe and Mg, and low in Ti, unlike any known lunar basalt, and may be derived from material of pyroxenitic composition in the Apennine Front.

Metal grains in Apollo 12 igneous rocks

Abstract The Apollo 12 igneous rocks we have examined contain abundant disseminated grains of metallic iron with a wide range of Ni and Co contents. The compositions of these metal grains vary to a degree not found in terrestrial igneous rocks or in meteorites. Ni and Co are generally higher in metal grains enclosed by the earliest-formed minerals (olivine and chromite) and are lower in those associated with later phases. The metal grains do not indicate the former presence of an immiscible metal liquid but have formed by reduction from the silicate melt. Formation of the metal grains accompanied, and may be a direct result of, the crystallization of the major phases under low oxygen partial pressures. The crystallization of chromite, for example, from a melt containing divalent chromium, may have resulted in the reduction of ferrous iron to metal. The presence of nickel-iron grains in lunar igneous rocks indicates that metal grains in lunar soil need not be exclusively of meteoritic origin.

Highly aluminous glasses in lunar soils and the nature of the lunar highlands.

Abstract Approximately 25 per cent of the glasses in two Apollo 14 soil samples and in the soils at two levels in the Luna 16 core have compositions equivalent to anorthositic gabbro. Reassessment of the non-mare glass components in the Apollo 11 and 12 soils shows that glasses with the composition of anorthositic gabbro are common to both; gabbroic anorthosite glasses are less common, and anorthositic glasses, rare. Anorthositic gabbro glasses have the same major element composition at all four sites, and resemble the Surveyor 7 analysis from a ‘highland’ site. Thus, strong presumptive evidence exists that material with this specific composition is abundant in the lunar highlands.

Petrology of a portion of the Mare Fecunditatis regolith.

Abstract 1300 microprobe analyses of glasses, pyroxenes, feldspars, oxides, olivines, troilite and metal in two 0.025 g samples of the Luna 16 return were made in order to characterize the Mare Fecunditatis regolith. Pyroxenes display a very wide compositional range, extreme fractionation, and metastable crystallization. Solid solution of Ti, Al, and Cr is appreciable and most pyroxenes plot along an Al:Ti line 2:1, similar to Apollo 11 clinopyroxenes. Orthopyroxenes are very rare. Zoning in plagioclase is varied but not extensive; compositions from An75 to An100 are dominant. The compositional distribution is indistinguishable from Apollo 12 and 11 low-K basaltic plagioclases. No potassic feldspars were found. Ilmenite is the dominant oxide phase, with minor ulvospinel, rare chrome spinel and spinel. The latter resemble Apollo 14 pink spinels. Olivines range from Fo75 to Fo11 but the majority range from Fo60 to Fo70 thus more iron-rich than olivines from other maria. On the basis of preferred compositions, a tentative classification of glasses has been made. Twenty-three percent of the glasses are Al-rich, Fe, Cr-poor, have Ca/Al ratios similar to many Apollo 14 glasses and are considered to have a non-mare origin. Their compositions are essentially the same as that of the high Al component at all Apollo landing sites. Glasses equivalent in composition to Fra Mauro basalts (KREEP) and to granite are extremely rare. The majority of glasses, mare-derived, are substantially higher in Fe, Ti, and Cr and lower in Ca and Al. They are divisible into a major group, Fecunditatis type A basaltic glasses, with less than 5% TiO2, and a smaller group, Fecunditatis type B basaltic glasses, with more than 5% TiO2. The type A glasses are richer in Al, and lower in Fe than glasses at the Apollo 11 or 12 sites. Type B glasses are similar to the high Fe basaltic glasses from the Apollo 11 regolith. If the type A glasses reflect the characteristic basalts at the Mare Fecunditatis site, then these are intermediate in major element chemistry between Apollo 11 and 12 basalts and the aluminous non-mare basaltic rocks.

The Bholghati howardite: Petrography and mineral chemistry

Abstract A 10 g sample of the Bholghati howardite was disaggregated in order to separate two eucrite clasts, several small carbonaceous clasts, fragments of diogenitic pyroxene, and bulk matrix. The eucrite clasts show evidence of moderately rapid cooling from a melt, followed by prolonged subsolidus annealing. The carbonaceous clasts mostly resemble CM2 carbonaceous chondrites with low-iron silicates and Fe-Ni sulphides in a fine grained dark matrix. One clast, however, is mineralogically, petrographically, and compositionally similar to a CI 1 chondrite. Both carbonaceous and eucritic clasts have a complex history prior to incorporation into the howardite matrix with no evidence of significant metamorphism since assembly. Most clasts in the howardite breccia are monomineralic, with pyroxene and plagioclase pre-dominant. Pyroxenes range from ‘diogenitic’ to ‘eucritic’ with diogenitic compositions most abundant; a significant number of intermediate compositions are present, consistent with derivation from a series of rocks related by fractionation.

Luna 20 soil - Abundance and composition of phases in the 45-125 micron fraction.

Abstract Glass compositions in the Luna 20 soil indicate a minor contribution of mare rocks and a major contribution of highly feldspathic highland material. Glasses with the composition of Highland basalt (anorthositic gabbro or norite) predominate in a range of highly aluminous glasses. The analyses of minerals in the soil show that the highland rocks have a unique assemblage of minerals that can readily be distinguished from the mineral assemblages of either mare or KREEP basalts. The soils are characterized by abundant anorthitic (An 92–99 ), low-Fe plagioclase. Highly magnesian orthopyroxenes, pigeonites and augites are the most prominent pyroxenes. Unlike mare basalt pyroxenes, clinopyroxenes with intermediate Ca values are not abundant, but extreme iron enrichment towards pyroxferroite does occur. Olivines are more abundant than at other sites and are Mg-rich, low in Ca and Cr. Spinels with compositions approaching MgAl 2 O 4 predominate over pleonastes and chromites. Ilmenite and metal are present but not abundant. These data establish the unique nature of the minerals in the highland soils. The mineral compositions are consistent with derivation from a suite of highly feldspathic rocks in which Highland basalt compositions predominate. Some of the mineral data, particularly from the pyroxenes, are suggestive of surface or near-surface processes, rather than plutonic crystallization.

A Chondrule in the Chainpur Meteorite

The occurrence of glass as a major constituent in a chondrule from the Chainpur meteorite provides evidence that the chondrules formed by rapid cooling of liquid droplets. The virtual absence of nickel in the silicates suggests that it segregated into the metal phase in the molten stage, prior to crystallization of the silicates.

Olivine and pyroxene in the Orgueil meteorite

Abstract The Orgueil meteorite contains angular unaltered grains of dominantly low-iron olivine and pyroxene. Their presence in a matrix of hydrous low-temperature silicates is evidence that Orgueil is a low-temperature mechanical mixture.

Coexisting bronzite and clinobronzite and the thermal evolution of the Steinbach meteorite

Abstract Steinbach is a stony-iron meteorite with approximately equal amounts of silicate and metal that shows Widmanstatten structure. The silicate portion contains tridymite, orthobronzite, and clinobronzite that formed by inversion from high-temperature protobronzite. The assemblage orthobronzite-protobronzite-tridymite-metallic iron indicates an equilibrium temperature of 1200°C and an ƒo2 of 10−12 under a total pressure of less than 2 kbar. Preservation of the high-temperature phase relations implies much more rapid cooling in the 1200-700°C range than the rates that have been deduced for the development of Widmanstatten structure in the 700-500°C range.